envvar

envvar is a tiny JavaScript package for deriving JavaScript values from
environment variables.

``javascript
const envvar = require('envvar');

const GITHUB_API_TOKEN = envvar.string('GITHUB_API_TOKEN');
const HTTP_MAX_SOCKETS = envvar.number('HTTP_MAX_SOCKETS');
const ENABLE_FEATURE_X = envvar.boolean('ENABLE_FEATURE_X', false);
`

If one argument is provided the environment variable is __required__. If the
environment variable is not set, an envvar.UnsetVariableError is thrown:

UnsetVariableError: No environment variable named "GITHUB_API_TOKEN"

If two arguments are provided the environment variable is __optional__. If the
environment variable is not set, the default value is used. The default value
must be of type Boolean in the case of envvar.boolean, of type Number in the
case of envvar.number, or of type String in the case of envvar.string. If
it is not, a TypeError is thrown.

The value of the environment variable must be the string representation of a
value of the appropriate type: for envvar.boolean the only valid strings are
'true' and 'false'; for envvar.number applying Number to the string
must not produce NaN. If the environment variable is set but does not have
a suitable value, an envvar.ValueError is thrown:

ValueError: Value of process.env["HTTP_MAX_SOCKETS"] does not represent a number

$3

This is similar to envvar.string, but with constraints. There may be a small
number of valid values for a given environment variable. For example:

`javascript
const NODE_ENV = envvar.oneOf('NODE_ENV', ['development', 'staging', 'production']);
`

This states that process.env.NODE_ENV must be set to development,
staging, or production.

A default value may be provided:

`javascript
const NODE_ENV = envvar.oneOf('NODE_ENV', ['development', 'staging', 'production'], 'production');
`

This states that process.env.NODE_ENV must either be unset (in which case the
default value is assumed), or set to development, staging, or production`.